Removing mandrel from PTFE 1

[JPC10]

I'm reflowing pebax's over a PTFE liner with a mandrel inside. Having problems with particulate when removing the liner. Looks like the liner is being torn when pulling out the mandrel manually with a clamp (Requires a bit of force). The mandrel OD is 0.0005 smaller than the ID of the liner. Have tried 0.001 smaller as well but still getting the particulate problems. Currently trying to smoothen mandrel surface with 800 grit abrasive paper and trying to run at a lower reflow temp.

Any suggestions would be appreciated
JPC

 

[patprimmer]

Polish the mandrel in line of draw, not in a lathe.

Push the tube off the mandrel, do not pull the mandrel out. Think of the Chinese finger trap principle.

If you can lift the tube off the mandrel with some air pressure that will certainly help.

A lubricant on the mandrel that will withstand the coating process temperatures will help. This might be a silicon mould release spray, some proprietor mould release wax, or simply being dusted with a metal stearate powder, like sodium or calcium stearate.

Regards

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[JPC10]

Thanks Patprimmer,
Can't use any lubricants (medical). Reducing the temps, seems to have helped a bit but still getting particulate. Will try use air cooling on mandrel/tubes.

 

[GilPolymers]

Hello,

I don't really know how that mandrel is, but can't you just make a different type of mandrel?!
Try to design a 4 parts mandrel... so you can assemble and disassemble it inside the part. I didn't understand quite well the question but this can hopefully help!

Regards,

Pedro

 

[JPC10]

Hi Pedro,
I'm currently putting a PTFE liner onto a mandrel, then putting 3 different types of pebax over the PTFE, covering the Pebaxs with FEP and then reflowing the pebaxs so that they join together using a vertical heat shrinker machine. When the part is cooled I remove the mandrel. Removing the mandrel is tearing the PTFE. I've tried using stainless steel & PTFE coated mandrels, with outer diameter ranging from 0.001" & 0.0005" smaller than the ID of the PTFE. The mandrel are hard to remove. Running at lower temps seems to be better(easier to remove) but still getting the particulate problem. The ID of the liner is 0.049". How this help you understand.

Thanks for your idea.

JPC10

 

[Pud]

Try a bright chrome plated mandrel.


Cheers


Harry

 

[patprimmer]

Can't you get medical grade lubricants or mould release. I would have thought you could get medical grade zinc or calcium stearate. After all they are only saponified beef fat.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[JPC10]

Have tried coating the mandrel with silicon but still no luck. PTFE is etched, so thinking it might be causing problems (might try larger wall thickness to improve the strength), also the compression for the FEP might be a little excessive causing the pebax/liner to adhere to the mandrel. Will look into all suggestions.

Thanks for all the replies.

 

[patprimmer]

PTFE has very poor compressive strength and will easily conform to the surface of the mandrel.

The material you are coating it with will shrink and cause the PTFE to be pressed against the mandrel as a tight fit.

heating the mandrel as hot as possible just might help as it will then shrink as it cools reducing the effect of the shrinkage of the coating, but it will also soften the PTFE and maybe allow it to conform more. It will also need to be cooled before removal. An aluminium mandrel might be best re cooling time and expansion rate.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[JPC10]

Still having no luck, even heating mandrel. Tried relaxing in oven as well. Waiting on different mandrels to come to try out.

Thanks

 

[patprimmer]

Have you draw polished the mandrel.

Are you pushing the tube off with a sleeve that fits snug but not tight on the mandrel.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[JPC10]

Have draw polished the mandrel. Our current method of removing the mandrel involves placing mandrel through a small hole that acts as an end stop for the pebax tube. Clamp the mandrel and manually pull it out on a slide. (requires a small amount of force). Have also tried clamping the whole length/piece of tube to give uniform compression and pull the mandrel out with a chuck/clamp hooked up actuator/linear slide. Still tearing the liner.
Should have different mandrels in soon to try out.

Thanks again
JPC